This invention relates to electric switches and more particularly to such switches which are used to switch very large direct currents as are found in electromagnetic projectile launching systems.
One well-known type of electromagnetic projectile launching system includes a power supply which comprises the series connection of a direct current homopolar generator and an inductive energy storage device. This power supply is connected to the breech ends of a pair of generally parallel projectile launching rails and a firing switch is connected to short across the breech ends of the rails. In operation, the rotor of the homopolar generator is spun up to a desired speed before its brushes are dropped. With the firing switch in a closed position, the generator brushes are dropped and current flow is established through the inductor. Thus, part of the generator energy is transferred to the inductor. Subsequently, the firing switch commutates current into the projectile launching rails and through a sliding armature positioned between the rails to place a high acceleration force on an associated projectile.
Certain abnormal conditions may result in an undesirable effect wherein a voltage reversal appears across the generator terminals. This may cause the generator to rotate in the reverse direction. Furthermore, other possible component malfunctions may occur such as the failure of the firing switch to open on command. Under such conditions, the energy stored in the homopolar generator must be dissipated within the inductor-charging loop, which includes the homopolar generator brushes. This may create mechanical problems such as excessive brush wear as well as thermal heating of the components.
One proposed solution to these problems is to include two switches, a crowbar switch and a dump switch, in the system. The crowbar switch, which is normally open, is connected in parallel with the homopolar generator-inductor assembly and upon actuation provides a by-pass for current which normally flows through the firing switch or through the launcher rails. This prevents overheating of the firing switch and the launcher rails under abnormal conditions. The dump switch is connected in series with the homopolar generator, is normally closed, and has a dump resistor connected across its terminals. The opening of the dump switch would force current to flow through the dump resistor, thus dissipating energy stored in the homopolar generator and the inductor.
To minimize damage, it is preferred to activate simultaneously with the crowbarring or following it immediately, the dump switch so that an electric arc is generated. The arc voltage then can force the current to flow through the dump resistor, thereby starting energy dissipation. To have an optimum performance in energy dissipation and to prevent generator voltage reversal, a fast-acting switch mechanism with low jitter and precise coordination of actuation between the crowbar and dump switches is required. It is therefore desirable to develop a simpler system which can protect the homopolar generator and other components from unacceptably high resistive heating while preventing voltage reversal across the generator terminals.